Orbital abrading machines are well-known and generally comprise a portable, manually manipulatable housing, a motor supported by the housing and having or being coupled to a drive shaft driven for rotation about a first axis, and an assembly for mounting a pad for abrading a work surface for orbital movement about the first axis. In a random orbital abrading machine, the assembly serves to additionally mount the pad for free rotational movement about a second axis, which is disposed parallel to the first axis.
The assembly typically includes a head portion coupled for driven rotation with the drive shaft about the first axis and defining a mounting recess having an axis arranged coincident with the second axis, a bearing supported within the mounting recess, and means for connecting the pad to the bearing for rotation about the second axis.
Orbital machines by nature are subject to dynamic unbalance and require the inclusion of a counterbalance system to reduce vibration to an acceptance level. The typical design approach has been to account only for the unbalance, which is created by the mass centers of the pad and portions of the assembly not disposed concentric to the first axis, by the addition of balancing masses to the housing. This approach can create a machine that is balanced, that is, has acceptably low vibration levels, while the machine is running at free speed in an unloaded condition. However, once the machine is loaded, as a result of placing the pad in abrading engagement with a work surface, additional forces are introduced and the machine becomes unbalanced and this unbalance is detected by the operator in the form of vibration. This is undesirable and in severe cases, may lead to vibration induced injuries such as carpal tunnel syndrome and white finger.
The counterbalance system referred to above, which may be used in the design of both orbital and random orbital machines, is described for example in Chapter 12 of Mechanisms and Dynamics of Machinery, Third Edition, by Hamilton H. Mabie and Fred W. Ocvirk, published by John Wiley & Sons.
Another approach is that adopted for the Atlas Copco Turbo Grinder GTG40, which uses an SKF Nova AB auto-balancing unit to reduce vibration under various loading conditions. This unit features the use of a plurality of ball bearings, which are arranged within an annular raceway and free to move therewithin as required to reducing vibrations.